A new study makes clear the costs of legalization.

By 2011, Maastricht, a pleasant town in the southern Netherlands, was worried that the marijuana legally sold in its coffee shops was attracting too many drug tourists. To address the concern, the city restricted purchase of marijuana to Dutch, Germans, and Belgians. This new law gave researchers Olivier Marie and Ulf Zölitz a useful opportunity: By examining the performance of international students at a Maastricht university before and after the change was made, they could determine the effect that marijuana had on academic performance.

The results, Marie and Zölitz found, suggest that the drug is not quite as harmless as its proponents tend to claim. Banning open access to marijuana increased students’ grades by an average of about 11 percent of a standard deviation. Note that this calculation considers the entire body of foreign students who were denied access, not just the regular marijuana users. The effect was stronger for low performers and women, where it was closer to 13 percent of a standard deviation. Perhaps unsurprisingly, the difference was far more pronounced in quantitative courses, where the reduced access affected grades by an average of 23 percent of a standard deviation.

The researchers note that this is a substantial effect, comparable to having a professor “whose quality is one standard deviation above the mean” or a roommate with a one-standard-deviation higher GPA. In fact, legalizing marijuana has about the same negative effect on academic performance as that of legalizing access to alcohol.

This is not necessarily an argument against the movement to legalize marijuana, which has been gaining steam in the United States. We legalize alcohol, after all. Giving people the choice to make bad decisions is very often the right thing to do, and it is shameful to lock people up for behavior that most Americans do not even consider morally wrong.

But let’s not pretend legalization would exact no costs. What this study shows is that it would probably hurt low-performing and unmotivated students and encourage more people to smoke for the first time. It may also keep more workers out of the labor force — particularly male ones — as previous studies have observed.

If we accept that marijuana is not a moral issue and should be legalized, the question then becomes what the optimal social attitude toward it should be. Given the above evidence of its negative effects, we should adopt a position of stigmatizing anything more than light usage.

Consider that most people would not judge it immoral to smoke habitually or to eat poorly and shun exercise, yet there is still some measure of social censure for both. These behaviors are harmful to society, we reason, and ought to be discouraged. Obesity rates are slowing now and cigarette consumption has dropped dramatically over the last several decades. There are probably many reasons for both trends, but social censure almost certainly plays some role.

We should take up a policy of cautious neutrality towards moderate, responsible marijuana use and an attitude of stern condemnation toward anything further.

And there are a host of other activities that have little to do with morality and are clearly permissible in moderation but represent at the same time dangerous temptations. There is evidence that video games are responsible in part for the declining labor-force participation of young men. Internet pornography may be discouraging marriage. It is probably possible to become addicted to both. These problems, like marijuana dependence, will almost certainly be concentrated in communities that are already under enormous strain from the collapse of traditional social institutions, as Charles Murray exhaustively documents in his book Coming Apart. It won’t be upper-middle-class liberals who will be hurt by widespread access to marijuana or increasingly addictive video games or forms of pornography. Rather, it will be young men, and to a lesser extent young women, who grow up in families without the resources, both economic and cultural, to ensure that they finish college, enter the job market, and get married.

Yet there is little appetite in affluent communities for enforcing the sort of stigmas that could help establish norms of healthy behavior. Many will quietly agree that it’s pathetic to play video games, watch porn, or smoke pot all day, but the live-and-let-live attitude tends to triumph over such judgments in public spaces. At times, it even morphs into outright endorsement of these activities. This is a mistake, particularly if we are going to go down the uncertain road of legalizing marijuana. At the very least, we should take up a policy of cautious neutrality toward moderate, responsible use and an attitude of stern condemnation toward anything further. Such behaviors are not okay, we should say; they do not reflect a worthwhile variant of the good life; they are not an acceptable model for how one ought to live. We court disaster otherwise.